Effects of predation risk on life history strategies in marine plankton

This thesis explores how different aspects of life history strategies in common marine planktonic organisms change when they perceive a chemical cue of predation risk. Predators can have dramatic effects on prey organisms by inducing responses only by their presence. Such induced defenses can actually have stronger effect on a community than the consumption itself, as the defenses influence a larger proportion of the prey and may create cascading effects in the food web. Despite their huge ecological importance much is still unknown about how marine plankton adjust life history traits according to prevailing predation pressure. Here, I have looked into some life history traits in marine plankton when experimentally exposed to predator chemical cues. Three studies focus on copepods, the dominating group of zooplankton. Importantly, my findings suggest that marine copepods do respond to predator chemical cues by altering several key life history traits. Paper I shows an effect of perceived predation risk on growth and development on naupliar stages of a common coastal species. In paper II non-consumptive effects on egg-carrying were demonstrated. Paper III documents significant costs in male copepod spermatophore production. Perceived risk generally altered swimming behavior and also feeding rates under food limited conditions. All in all, papers 1-III demonstrate that vital life history traits in copepods, like growth, development and reproductive investments by both females and males are to some extent regulated by predation risk. Paper IV considers chain length plasticity in a diatom, one of the most ecologically important phytoplankton groups. Based on experiments and an empirical model, we show that chain length plasticity in a common diatom species is consistent with a grazer avoidance strategy. Altogether, the results presented in my thesis suggest that predation risk is one of the mechanisms that regulate fundamental life history traits in marine plankton